Optical fibers are commonly employed for communicating data at high bandwidths. They have been viewed by the telecommunications industry as a crucial ingredient for evolving the communications infrastructure to its current state, and for that reason the investments to develop fiber optic communications technology have focused on making such bandwidths available over long distances. The long distances further necessitate producing cables that are simultaneously affordable and robust. Telecommunications cables typically must resist not only the minor traumas associated with transport and installation, but also the insidious effects of aging and long term exposure to the elements.
As one example, consider the well-known hydrogen darkening effect. Over a long exposure time, hydrogen, whether arising from corrosion, biological processes or other water-associated causes, migrates into the fiber core and reacts chemically with dopants or other impurities to “tint” the glass. Over short runs, the effect of this tint may be barely perceptible, but over long distances the signal is overwhelmed by the resulting attenuation. Two other water-related degradation mechanisms are stress corrosion and zero-stress aging, both of which, over time, reduce the strength and transparency of unprotected optical fibers.
To combat these effects and improve communications cable robustness, the industry has, through a great deal of investment and effort, developed a standard design approach. To protect optical fibers from hydrogen darkening and water-related degradation mechanisms, communications cables route optical fibers through hermetically sealed stainless steel tubing. In the event of a pinhole or other flaw in the stainless steel tubing that might permit water to breach the barrier, gel or another filler material in the tubing serve to prevent fluid migration along the length of the tubing. Often, the stainless steel tubing is itself coated to provide a redundant seal against water penetration, particularly in high pressure or marine applications.
Rather than reinventing the wheel, the geophysical surveying industry has leveraged the technology developed by the telecommunications industry for buried cables and subsea cables. Designs in development for optical-communication-based survey streamers and seafloor cables typically employ commercially available subsea telecommunications cables and technology as the backbone of system designs. Despite the manufacturing difficulties and costs caused by the standard design approach, these inherited precautions against exposing the optical fibers to water represent the industry's accepted wisdom: common sense shared by technicians, engineers, and experts alike.